Electrodeposition coating compositions and methods are widely used in industry today. During electrodeposition, an ionically-charged resin having a relatively low molecular weight is deposited onto a conductive substrate by submerging the substrate in an electrocoat bath having dispersed therein the charged resin, and applying an electrical potential between the substrate and a pole of opposite charge (usually the walls of the vessel holding the bath). This produces a relatively soft coating on the substrate of low molecular weight resin. This coating is usually converted to a hard high molecular weight coating by curing or crosslinking the resin.
One curing mechanism utilizes a melamine formaldehyde resin curing agent in the electrodepositable coating composition to react with hydroxyl functional groups on the electrodeposited resin. This curing method provides good cure at relatively low temperatures (e.g., 132.degree. C.), but the crosslink bonds contain undesirable ether linkages and the resulting coatings provide poor overall corrosion resistance as well as poor chip and cyclic corrosion resistance.
In order to address some of the problems with melamine-crosslinked electrocoats, many users employ polyisocyanate crosslinkers to react with hydroxyl functional groups on the electrodeposited resin. This curing method provides desirable urethane crosslink bonds, but it also entails several disadvantages. In order to prevent premature gelation of the electrodepositable coating composition, the highly reactive isocyanate groups on the curing agent must be blocked (e.g., with an oxime or alcohol). Blocked polyisocyanates, however, require high temperatures (e.g., 150.degree. C. or more) to unblock and begin the curing reaction. The resulting electrocoats can also be susceptible to yellowing. Moreover, the volatile blocking agents released during cure can cause other deleterious effects on various coating properties, as well as increasing VOC. There is thus a need in the art for electrodepositable coating compositions that could provide desirable urethane crosslink linkages, but avoid the problems that accompany the use of blocked polyisocyanate curing agents. Also, since melamine and polyisocyanate curing agents for hydroxy-functional resins are also used in a variety of coating compositions other than electrocoat, and still entail the same sets of disadvantages, this need is observed throughout the coatings art.